try fix travis flakiness with CODE_SIGNING_REQUIRED=NO

.travis.yml

     - ANDROID_HOME=$HOME/android-sdk-macosx
     - PATH=$PATH:$ANDROID_HOME/tools:$ANDROID_HOME/platform-tools
     - YARN_CACHE=$HOME/.yarn-cache
+    - CODE_SIGNING_REQUIRED=NO
 
 branches:
   only:

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/NavigationControllerTest.java

 		assertThat(uut.size()).isEqualTo(2);
 		assertThat(uut.handleBack()).isTrue();
 		assertThat(uut.size()).isEqualTo(1);
+		assertThat(uut.handleBack()).isFalse();
 	}
 }